Organocatalysis thiourea

Even if organocatalysis is a common activation process in biological transformations, this concept has only recently been developed for chemical applications. During the last decade, achiral ureas and thioureas have been used in allylation reactions [146], the Bayhs-Hillman reaction [147] and the Claisen rearrangement [148]. Chiral organocatalysis can be achieved with optically active ureas and thioureas for asymmetric C - C bond-forming reactions such as the Strecker reaction (Sect. 5.1), Mannich reactions (Sect. 5.2), phosphorylation reactions (Sect. 5.3), Michael reactions (Sect. 5.4) and Diels-Alder cyclisations (Sect. 5.6). Finally, deprotonated chiral thioureas were used as chiral bases (Sect. 5.7). 

Scheme 12.19 Organocatalysis with an activated thiourea derivative (Acat/Auncat = 8 at 1 mole % in CHC13).

Asymmetric cyanohydrin synthesis remains an important reaction for organocatalysis and many of the catalyst classes discussed in subsequent chapters give highly effective catalysts for this reaction. These include Cinchona alkaloid derivatives, thioureas, guanidines, amine-oxides, diols and diamines. 

Zhang, Z., Lippert, K. M., Hausmann, H., et al. (2011). Cooperative Thiourea-Brpnsted Acid Organocatalysis Enantioselective CyanosUylation of Aldehydes with TMSCN. J. Org. Chem., 76(23), 9764-9776. 

Axial-to-central chirality transfer in cyclization processes 13CSR8434. Bifunctional primary amine-thioureas in asymmetric organocatalysis 13OBC7051. 

A quantitative approach to nucleophilic organocatalysis 12BJ01458. Selective photocatalytic reactions with organic photocatalysts 13CS561. Terpene-derived bifunctional thioureas in asymmetric organocatalysis 13CAC2756. 

To circumvent the potential problems associated with the use of metallic catalysts in the area of biomedical applications, organocatalysis was first investigated by Iwasaki et al using l,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and l,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD). This procedure was successfully used by other groups to elaborate complex PPE architectures. Clement et al. improved the procedure by using a combination of thiourea and DBU that was found to be an effective... 

A pioneer in the field of the asymmetrie (thio)urea organocatalysis was Eric Jacohsen, who first reported a chiral (polymer-hound) Schiff base thiourea derivative for asymmetric Strecker reactions optimised from parallel synthetic libraries/ These catalysts can be used either in solution or immobilised to a polystyrene resin, with the latter retaining efficiency, after repeated recycling/ The key factors responsible for high enantioselectivities were the presence of bullqr substituents at both the amino acid position and at the 3-position of the aromatic ring (Scheme 19.3). 

Dove, A.P., Pratt, R.C., Lohmeijer, B.G.G., Waymouth, R.M., Hedrick, J.L., 2005. Thiourea-based bifunctional organocatalysis supramolecular recognition for living polymerization. Journal of the American Chemical Society 127, 13798—13799. 

Keywords Transfer hydrogenation Organocatalysis Hantzsch ester Trichlorosilane Phosphoric acid Aminocatalysis Thioureas Lewis bases Reduction Hydrosilylation... 

Asymmetric Organocatalysis Introducing a Thiourea Catalyst for the Petasis Reaction... 

The applications of primary and secondary amine-ureas and -thioureas in asymmetric organocatalysis have been reviewed (138 references), as has the use of oxazolidinones as chiral auxiliaries in asymmetric aldols employed in total synthesis (193 references). 

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